It is desirable to test spark plugs for automotive engines as a quality control measure in spark plug manufacturing, to assure engine quality at the time of engine manufacture, and to diagnose engine problems on vehicles already in service. In each case it is helpful to run the test in a manner which resembles actual operating conditions without introducing unknown or difficult-to-control variables.
In making a spark plug load test, the object is to detect any physical problems in the spark plug. It has long been recognized that spark plug tests must be run under load in order to reveal defects which cause problems under actual running conditions. Spark plug load refers to the electrical load or impedance that the spark plug presents to the high voltage ignition circuit.
Spark plug performance is controlled by two basic conditions. One is the physical (electrical) condition of the spark plug gap. The second is the physical condition within the spark plug gap itself at the precise instant that the ignition voltage is applied. These conditions are as follows:
Physical Condition of the Spark Plug Gap.
1. Size of the gap.
2. Condition of the electrodes: geometry and contamination.
3. Resistance of the insulation.
4. Cracks or deformities in the insulation.
5. Open electrode path.
Physical Conditions within the Spark Plug Gap.
1. Gas pressure at the instant of ignition. This pressure is dependent upon ignition timing, engine load, compression ratio and engine RPM.
2. The gas velocity passing through the spark plug gap at the instant of ignition.
3. Engine fuel. The actual composition of the fuel being fed into the gap.
4. The temperature of the gas.
5. The fuel/ratio at the instant of ignition.
In prior spark plug load testing it has been the practice to make measurements under actual engine running conditions with a constant engine load imposed by a dynamometer. To make the spark plug test as repeatable and therefore as valid as possible, it was necessary to control the condition of the fuel as much as possible. While it was possible to maintain a consistent fuel quality (in the manufacturing environment) the fuel/air mix and its velocity in the plug gap at the instant of applied ignition voltage always remained a problem. Because of those two variables, the spark plug load readings displayed a somewhat erratic pattern from reading to reading. To compensate for this lack of control, several sets of spark plug load readings were taken and then average prior to being displayed by the measuring instrument.
Some other variable conditions can be controlled during testing. These are engine speed, timing and torque. By keeping these three conditions as constant as possible and by using an averaged spark plug load, an acceptable level of test result validity can be achieved. Still, this test has limited application since the expense of a dynamometer precludes its use in many places where spark plug testing is desired in the manufacturing process.
Moreover, the prior test is wholly impractical for vehicle servicing at local garages.